The excellent charge carrier mobility of graphene suggests several applications in fast analog electronics, including transistors potentially operating in the terahertz (THz) regime, as well as graphene interconnects with low resistance and capacitance to replace copper. Furthermore, the ambipolar nature of transport in graphene, together with the high mobility of charge carriers, makes possible new non-linear device architectures, such as frequency multipliers, mixers, and modulators. These devices have potential application in radiofrequency (RF) and mixed-signal circuits. None of them requires a band gap in graphene or a high on-off ratio for switching, and thus such devices are independent of the extensive efforts to create a band gap in graphene to enable its use for logic applications. In addition to a high mobility, a high carrier concentration is preferable for optimum function of these devices. Thus maximizing mobility is not the only goal when investigating graphene conductors for these applications.
The realization of any graphene-based devices is also dependent on finding functional materials structures where the high conductivity observed for freestanding sheets of graphene is maintained. Mobility values of ˜105 cm2/Vs (T=300K) and ˜106 cm2/Vs (T=4.2K) have been measured at typical carrier concentrations n˜1011 cm−2 for exfoliated graphene suspended bridge-like and contacted at its ends. (K. Bolotin et al., Ultrahigh Electron Mobility in Suspended Graphene, Solid State Commun. 2008, 146, 351; and K. Bolotin et al., Temperature-Dependent Transport in Suspended Graphene, Phys. Rev. Lett. 2008, 101, 096802.) A suspension geometry imposes severe constraints on device processing and creates serious reliability issues. To overcome these limitations, several materials have been proposed and tested as hosts of grown and transferred graphene sheets, with the general conclusion that the carrier mobility of free-standing graphene is degraded by bonding to a substrate, in some cases quite significantly. The majority of studies have focused on graphene transferred onto SiO2, because of the desire for a good and inexpensive dielectric substrate. They produce mobility values in the range of 102-103 cm2/Vs at 300K and at a carrier concentration n˜1012 cm−2, two to three orders of magnitude lower than for suspended graphene at similar carrier concentrations. (V. E. Dorgan, M.-H. Bae, and E. Pop, “Mobility and saturation velocity in graphene on SiO2. Appl. Phys. Lett. 2010, 97, 082112.) Transport measurements in graphene transferred to h-BN substrates showed mobilities of ˜104-105 cm2/Vs at 300 K and ˜106 cm2/Vs at 10K, with carrier concentrations n˜1011-1012 cm−2. (Dean, C. R. et al. Boron nitride substrates for high-quality graphene electronics, Nat Nano 2010, 5, 722.) The latter mobilities appear to be the highest values so far achieved for supported graphene.